Activity dependent modulation of neurotransmitter release, called short-term plasticity, has been implicated in several forms of behavior and learning and memory. Although it is mediated by presynaptic mechanisms, short-term plasticity has been shown at many synapses to be target-cell specific. Thus synapses made by axons from the same cell type onto target neurons of different types can exhibit markedly different properties of short-term plasticity. This implies the existence of a retrograde signal from the postsynaptic neuron to the presynaptic cell which alters the structure and/or function of the presynaptic terminal depending on the identity of the postsynaptic target neuron. While it has been known for over 30 years that presynaptic function is target cell specific, the identity of this retrograde messenger(s) is still not known. The long term goal of this project is to determine the postsynaptic molecules that are responsible for target-cell specific short-term plasticity. The synaptic cell adhesion molecules neuroligin 1 and SynCAM have recently been shown to induce presynaptic differentiation in neurons when expressed in non-neuronal cells in co-culture. This shows that neuroligin 1 and SynCAM are sufficient to trigger presynaptic differentiation, even in the absence of other normal components of the postsynaptic terminal. While neuroligin 1 and SynCAM have each been shown to be sufficient to induce formation of presynaptic terminals in neurons (referred to here as hemisynapses) that are functional at a basic level, it is not known whether they are sufficient to enable presynaptic terminals of hemisynapses to have the same complex presynaptic properties that neuron-neuron synapses possess. In particular, has not previously been shown whether they are capable of evoked neurotransmitter release, and if so, whether they have a similar release probability and mechanisms of short-term plasticity as neuron-neuron synapses. In this proposal, we will develop the hemisynapse preparation and measure evoked neurotransmitter release and short-term plasticity, in order to test the hypothesis that the cell adhesion molecules that trigger synapse formation are also involved in the retrograde signaling that modulates the functional properties of presynaptic terminals, yet are not sufficient to induce formation of presynaptic terminals with all of the same properties of neuron- neuron synapses. This system will provide a simple but powerful assay for a large range of future experiments investigating the influences of specific cell adhesion molecules, extracellular matrix proteins, neurotrophins, secreted factors, and postsynaptic density components on the formation and function of presynaptic terminals. These experiments address fundamental questions of presynaptic function, synaptic specialization and development that are important to our understanding of circuits in the brain, and which will have implications for learning and memory as well as neurodegenerative diseases and developmental disorders that cause mental retardation. These experiments address fundamental questions of presynaptic function, synaptic specialization and development that are important to our understanding of circuits in the brain, and which will have implications for learning and memory as well as neurodegenerative diseases and developmental disorders that cause mental retardation. [unreadable] [unreadable]